1. Field of the Invention
The present invention relates to cathode active materials and lithium batteries using the same.
2. Description of the Related Art
In general, cathode active materials used in lithium batteries contain transition metal compounds or oxides thereof and lithium, such as LiNiO2, LiCoO2, LiMn2O4, LiFePO4, LiNixCo1−xO2 where 0≦x≦1, or LiNi1−x−yCoxMnyO2 where 0≦x≦0.5 and 0≦y≦0.5.
Among these cathode active materials, LiCoO2 is typically used in lithium batteries. However, LiCoO2 is relatively expensive and has a limited discharge capacity of about 140 mAh/g. In addition, when the lithium battery is in a charged state, Li is removed from LiCoO2 so that LiCoO2 turns into Li1−xCoO2, which is inherently unstable and unreliable.
To address these problems, improved cathode active materials, such as LiNixCo1−xO2 where x=1 or 2 and LiNi1−x−yCoxMnyO2 where 0≦x≦0.5 and 0≦y≦0.5 have been proposed. However, these cathode active materials do not entirely overcome the above described problems.
In consideration of electrochemical potential, cost, discharge capacity, stability, and toxicity of metal oxides, Mn is the most suitable first row transition metal atom for replacing Co in a cathode of a lithium battery. In addition, Mn oxides and Li—Mn oxides can have various structures such as one-dimensional structures, two-dimensional layer structures, and three-dimensional framework structures. Examples of such structures include alpha-MnO2, beta-MnO2, and gamma-MnO2. In general, even when lithium is intercalated or deintercalated, the structural integrity of these Mn oxides is not destroyed.
Accordingly, Mn oxides having many structures have been proposed as new cathode materials. In particular, in line with the demand for high-capacity batteries, a composite oxide has been suggested as an alternative. An example of such a composite oxide is a layered structure of xLi2MO3-(1−x)LiMeO2, where 0<x<1 and M and Me can be Mn. However, although such composite oxides having layered structures have high initial discharge capacities, they also have high irreversible capacities. That is, during initial charging, the Mn in the Li2MO3 of the composite oxide has an oxidation number of 4+ as shown in the reaction scheme described below. This means that Mn is not additionally oxidized, and thus that oxygen is decomposed together with lithium into Li2O. Then, during discharging, only lithium enters the composite oxide since the decomposed oxygen cannot reversibly enter the composition, and Mn is reduced to 3+. Accordingly, the initial charge/discharge efficiency of Li2MO3 is maintained at 50%.                (charging) Li2Mn4+O3−Li2O→Mn4+O2         (discharging) Mn4+O2+Li→LiMn3+O2         
To obtain high discharge capacity, the content ratio of Li2MO3 can be increased to 50% or more. In this case, however, initial irreversible capacity is also increased.